The invention relates to multi-layer thermal barrier coating systems and to a method for producing them.
Increasingly higher gas temperatures are sought in stationary and airborne gas turbines in order to increase the efficiency of these machines. For this reason, turbine components are provided with thermal barrier coatings that generally comprise yttrium-stabilized zirconium oxide (YSZ).
It is disadvantageous that such porous ceramics are porous for oxygen. A bonding agent coating, made of an MCrAlY alloy; iron, cobalt, or nickel being employed for the metal M, or made of an aluminide, protects the substrate from oxidation and simultaneously improves the adhesion of the ceramic coating. Surface temperatures on the components today can reach up to 1200° C. during continuous operation.
It is a disadvantage that, when there is rapid and frequent heating and cooling of the coatings, stresses occur due to different thermal expansion coefficients, and these stresses can lead to damage and to failure of the joint.
Frequently, thermal barrier coating systems fail in the vicinity of the bonding agent coating where the stresses are locally amplified by special geometric relationships due to the interface roughness.
This effect is amplified when an oxide coating forms between the bonding agent coating and the thermal barrier coating and the oxide coating has drastically less plasticity and a lower expansion coefficient than the bonding agent coating. This leads to spalling of the ceramic thermal barrier coating when the oxide coating attains a critical thickness.
A thermal barrier coating is known from DE 198 52 285 C1 that is made of a glass-metal/ceramic composite and that is gas-tight and thus can also prevent formation of an oxide coating. However, the corrosion and aging susceptibility of the composite in oxidizing atmospheres at temperatures above approximately 1000° C. significantly limits employment of these composites as thermal barrier coatings. Moreover, these composites are substantially more heat-conductive than YSZ, for instance, so that much greater coating thickness is required to attain an equivalent thermal barrier.
A thermal barrier coating is also known from DE 100 08 861 A1, which comprises a composite made of YSZ or a glass-metal composite coating that is 50-100 μm thick and another coating disposed thereover that has significantly lower thermal conductivity and higher temperature stability. It is a disadvantage that the adhesion within this composite is inadequate. As a rule, the coatings fail first at the interface between the composite coating and the coating disposed thereover. However, they can also fail within the composite coating.
The object of the invention is to provide a method with which it is possible to produce thermal barrier coatings that have a better service life at high operating temperatures than the prior art. The object of the invention is furthermore to provide a component having a temperature-resistant and aging-resistant thermal barrier coating.